In the field of telephony, multiple pushbutton switches, referred to as keys, are used in key telephone sets to select central office or intercom lines. Each pushbutton operates an actuator that, when depressed, closes an associated switch to make the selection. When two or more pushbuttons are depressed simultaneously, the associated lines are interconnected, and all the parties connected to those lines are able to speak with one another in a conference call. While it is permissible to interconnect two or more central office lines together or two or more intercom lines together, it is not permissible to interconnect a central office line and an intercom line. This is because central office lines work off a 48-volt battery, and intercom lines normally work off of 24-volt systems.
One way of preventing conferencing of central office lines with intercom lines is disclosed in the copending application of R. L. Beecher, H. J. Hershey and R. S. Zieles, Ser. No. 790,461, filed Apr. 25, 1977 now U.S. Pat. No. 4,130,744 and assigned to the assignee of the present application. The pushbutton actuators in the key described therein are arranged in a row with the central office line selecting actuators in one group and the intercom line selecting actuators in a second group. Each actuator has an interlocking member mounted thereon that includes a disk, and a row of lockout members are positioned alongside of the actuators in the same plane as the disks. The lockout members are respectively positioned between the disks on the adjacent actuators, and the disk on each actuator is adapted to engage and laterally displace an associated lockout member when the actuator is depressed. Because the lockout members are closely spaced, this results in the displacement of other lockout members.
The lockout members have three different shapes. Lockout members of a half triangular-like shape are positioned between adjacent central office line selecting actuators. This half triangular-like shape is such that only an inclined shoulder portion on one side of the lockout members is engageable by the disks on these actuators. The engagement with that shoulder portion results in the lateral displacement of the lockout members toward the group of intercom line selecting actuators.
Lockout members of a half triangular-like shape that is the reverse of the foregoing lockout members are positioned between adjacent intercom line selecting actuators. Thus, again only an inclined shoulder portion on one side of the lockout members is engageable by the disks on these actuators. The engagement with that shoulder portion results in the displacement of the lockout members toward the group of central office line selecting actuators.
Finally, a lockout member of a full triangular-like shape is positioned between the central office line selecting actuator and intercom line selecting actuator that are adjacent to one another. This shape is such that an inclined shoulder portion on one side of the lockout member is engageable by the disk on the central office line selecting actuator so as to be displaced toward the intercom line selecting actuators. In addition, an inclined shoulder portion on the other side of the lockout member is engageable by the disk on the intercom line selecting actuators so as to be displaced toward the central office line selecting actuators.
As a result of the foregoing relationships, when one or more central office line selecting actuators are depressed, the lockout members are laterally displaced to a position wherein the disks on the intercom lines selecting actuators are blocked by the shoulder portions of the lockout members associated therewith. Thus, no intercom line selecting actuator can be depressed at the same time that a central office line selecting actuator is depressed. In a like manner, when one or more of the intercom line selecting actuators are depressed, the lockout members are laterally displaced to a position wherein the disks on the central office line selecting actuators are blocked by the shoulder portions of the lockout members associated therewith. Thus, no central office line selecting actuator can be depressed at the same time that an intercom line selecting actuator is depressed.
The problem with this arrangement is that should a customer decide to have more intercom lines and fewer central office lines or vice versa, it is necessary to disassemble the key and change one or more lockout members in order to provide the necessary interlock feature. Consequently, this modification is more appropriately done at a service center rather than on the customer's premises.